1. Field of the Invention
The invention relates to a demodulating system of a phase modulate a signal (QPSK: Quad Phase Shift Keying) which transmits a digital data as a phase change of a carrier.
2. Description of the Prior Art
For the sake of explanation, a 4-phase modulated signal is described here as an example but it may be also applicable to 2-phase, 8-phase, 16-phase, 32-phase, and also more than 32-phase modulated signals.
FIG. 13 is a block diagram showing an example of a conventional 4-phase modulated signal demodulating system. In FIG. 13, a carrier reproduction PLL (Phase Locked Loop) 2 reproduces a clock synchronized with a carrier "a" of a signal which is inputted from a 4-phase modulated signal input terminal 1. Phase detectors 4a and 4b multiply a reference clock which is outputted from a phase shifter 12 by a 4-phase modulated signal which is inputted from an input terminal 1. A phase shifter 12 generates two kinds of clocks, each of which has the same frequency as a carrier frequency "a" and phase of .+-.45.degree., respectively. Each of comparators 13a and 13b outputs a signal "H" or "L" according to a signal provided from a respective phase detector 4a and 4b under a predetermined threshold level.
FIG. 14 shows examples of signals at different points of a conventional 4-phase modulated signal demodulation system. In FIG. 14,
a: a 4-phase modulated signal to be demodulated; PA0 b: a reference clock for demodulation; PA0 c: a reference clock for demodulation; PA0 d: a.times.b; PA0 e: a.times.c; PA0 f: a signal obtained after a signal d passed through a most appropriate and ideal LPF; and PA0 g: a signal obtained after a signal e passed through a most appropriate and ideal LPF.
A demodulating operation of a conventional 4-phase modulated signal is described in reference to FIGS. 13 and 14. A 4-phase modulated signal inputted into an input terminal 1 reproduces a continuous clock signal with no phase change which is synchronized with a carrier of the input signal in a carrier reproduction PLL circuit 2. A costas loop system and a multiply system are generally used for this clock reproduction system. A clock signal outputted from the carrier reproduction PLL circuit fc, as shown in FIG. 13 (Prior Art), is supplied to the phase shifter 12, which generates two signals b, c each having phases of +45.degree. and -45.degree., respectively, from the carrier reproduction PLL circuit signal fc. The reproduction carrier signals having the phase of +45.degree. and -45.degree. are inputted to respective phase detectors 4a, 4b as reference signals. The reference signals b, c are multiplied by the input carrier signal a, respectively, and the resultant signals d, e are generated, respectively, from the respective phase detectors 4a, 4b. The resultant signals d, e thus obtained are transmitted to a comparator 13 through LPF 29a, 29b, respectively. The input signals f, g are compared with a threshold value of a reference at a comparator 13a, 13b, respectively, and phase information of the input signals is convened to binary signals which are outputted from the data output terminals 6 and 7, respectively.
Binary signals are outputted from the data output terminals 6 and 7, respectively, and 4-phase information, which correspond to four phases of 0.degree., 90.degree., 180.degree. and 270.degree. of the carrier in the 4-phase modulated system, is obtained from the combination of the binary signals. Four-phase information obtained from data output terminals 6 and 7 represents demodulation information of the 4-phase modulated signal.
As explained above, a multiplication of a reference clock and an input signal is used for a phase detection in a conventional phase modulated signal demodulation system. Therefore, a noise in an input signal or a distortion in an input waveform affects a multiplication and deteriorates demodulation characteristics.
Also, a phase drift or a waveform distortion of clocks of .+-.45.degree. which are used as reference for phase detection affects a multiplication and deteriorates demodulation characteristics.
Since a multiplication result is converted to a direct current signal by the LPF, the LPF characteristics also have a great influence on demodulation characteristics.